This invention relates to an electrostatic attracting sheet which is used with a holding plate and is adapted to fix or secure paper sheets in the so-called X-Y plotters, copying machines or the like to the holding sheet for a desired length of time by means of the Coulomb force.
The known electrostatic holding apparatuses utilize the electrostatic Coulomb force in order to attract a paper sheet or other object to be held on the holding apparatuses. Said electrostatic holding apparatuses each comprises an insulating base layer, a pair of electrodes formed thereon and an electrostatic attracting sheet laminated on and covering the base layer and electrodes. The attracting sheet is of a predetermined conductivity or of a limited volume resistivity. In said apparatuses, positive and negative terminals from a power source are connected to the electrodes so as to impart electrostatic charges to the electrostatic attracting sheet. Electric connection between the electrodes and the terminals is to be turned off to peel the paper sheets away from the attracting sheet.
Some examples of the known attracting devices are disclosed in the Japanese Patent Publications TOKKOSHO-55-20830 and -57-58872.
The electrostatic attracting sheets in said known devices have a structure such that spherical particles of carbon black are dispersed and embedded in a sheet of plastics or synthetic rubber. The extrusion method is used to form the sheets.
However, it has been difficult to disperse the carbon black particles homogeneously in the plastics or synthetic rubber. The attracting sheets made by such a method are disadvantageous because their dispersion is not well controlled but is likely to be influenced by conditions of the mixing of carbon black and/or the forming of the attracting sheet. Thus, they cannot exert their attracting force in a stable manner. In particular, fine spherical particles of carbon black will form bead chains of a kind which is called "structure". It is vaguely assumed that electrons flow through the chains or repeatedly jump from the aggregate particles of carbon black to the surrounding high polymer phases. This mechanism of electron current makes it difficult to obtain an attracting sheet comprising carbon black and having the volume resistivity uniform over its entire surface.
Thus, the values of volume resistivity vary within a wide range notwithstanding a constant amount (% by weight) of carbon black. Therefore, well-controlled stable attracting force falling within a narrow range has not been acquired for the electrostatic attracting sheet.
Therefore, the inventions disclosed in the Japanese Patent Publications are directed to the improvement in which such disadvantages in the known art, are diminished by adding a stabilizing agent, a plasticizer, an anti-aging agent, a lubricant and/or other special additives to the plastics of the attracting sheet. Such additives added to the electrically conducting material are intended to eliminate the bad influences of ambient temperature or humidity in order to stabilize the attracting force.
In spite of such an intention, a well stabilized attracting force of a desirable level has not been achieved up to now. Besides, such special additives have caused a considerable rise in manufacturing cost.
On the other hand, it has been tried to manufacture a white electrostatic attracting sheet by dispersing in the plastics titanium dioxide powder which are coated with tin. The particles are however almost spherical fine crystals, so that they cannot be homogeneously dispersed in such an amount that a satisfactory conductivity is enhanced to the plastics sheet.